Results of the PISA 2018 tests show that the average score in mathematics of the Spanish students is below the European Union and the OECD averages. These differences may be caused by factors related to the student or by those related to the school the student belongs to. The aim of this work is to identify the variables causing the differences in the mathematical performance of the Spanish students. Taking into account its hierarchical structure, a methodology based on the two-level multilevel regression has been applied to the Spanish data of the PISA 2018 study. According to our results, the socioeconomic and cultural index, the gender of the student, the repeater status, being immigrant, the language spoken at home and the student-teacher ratio have a significant effect on the mathematical performance of the students.